FCC has been, and will remain for quite some time, the primary conversion process in oil refining. In a typical present-day FCC process, a liquid feed mixture is atomized through a nozzle to form small droplets at the bottom of a riser. The droplets contact hot regenerated catalyst and are vaporized and cracked to lighter products and coke. The vaporized products rise through the riser. The catalyst is separated out from the hydrocarbon stream through cyclones. Once separated, the catalyst is stripped in a steam stripper of adsorbed hydrocarbons and then fed to a regenerator where coke is burned off. The products are sent to a fractionator for fractionation into several products. The catalyst, once regenerated, is then fed back into the riser. The riser-regenerator assembly is heat balanced in that heat generated by the coke burn is used for feed vaporization and cracking. The most common FCC feeds by far are gas oils or vacuum gas oils (VGO) which are hydrocarbon mixtures boiling above about 650.degree. F. When refiners need to convert heavy, or highly contaminated oils such as resids, they usually blend a small amount of such heavy oils with the gas oil feeds. Due to a dwindling supply of high-quality crudes, the trend in the petroleum industry is that FCC will have to convert more and more heavy, dirty feeds. Such feeds contain a high level of contaminants such as nitrogen, sulfur, metals, polynuclear aromatics, and Conradson Carbon Residue (CCR, a measure of asphaltene content). Hereafter, the term heavy component is used to include such highly contaminated hydrocarbons as resids, deasphalted oils, lube extracts, tar sands, coal liquids, and the like. Such heavy components are added to other feeds containing less heavy components to obtain an FCC feed. These heavy components will become a significant portion of FCC feeds in years to come.
The technical problems encountered with FCC feeds containing heavy components have been reviewed by Otterstedt et al. (Otterstedt, J. E., Gevert, S. B., Jaras, S. G., and Menon, P. G., Applied Catalysis, 22, 159, 1986). Chief among them are high coke and gas yields, catalyst deactivation, and SO.sub.x in flue gas. The coke forming tendency of such heavy component-containing feeds has traditionally been gauged by their CCR content. VGO feeds typically contain less than 0.5 wt % CCR, whereas atmospheric and vacuum resids typically contain 1 to 15 wt % and 4 to 25 wt % CCR, respectively. Since cracking of such heavy components can produce coke levels far higher than that required by existing FCC units, the maximum permissible level of the heavy component in the FCC feed is often limited by the unit's coke burning capacity. Many FCC units today are capable of cracking only 5 to 15 wt % resid, or heavy component, in the feed. Due to feed cost considerations, there is a strong need for economical methods that can expand the FCC's operating envelope, that is, to be able to increase the heavy component limit within existing hardware constraints.
What is needed in the art is an FCC method which allows for increased use of alternative feeds and yield improvements for desired products via stretching the operating limits of existing hardware.